The present invention relates to a method and an APPARATUS for gas separation after cooling a raw gas.
Gas-separation techniques for extracting product gas from a raw gas by cooling the gas and thereafter supplying the same to a gas separator section for rectifying separation or absorbing-separation process have been heretofore known. In particular, air separators for separating nitrogen, oxygen and argon etc., from air provided as a raw gas, which liquefy air and thereafter rectify the liquefied air, are now used in various fields.
Gas separators of this type necessitate compressing and expanding processes of a raw gas in accordance with operating conditions and, hence, require installations such as a compressor, an expansion turbine or the like. This type of separator in continuously operated for a long period of time in most cases, so that it is most important to consider how to reduce operation cost of, e.g., electric power consumption.
Many techniques have been developed for this theme. One known type of these techniques employs an expansion turbine with a compressor, namely, a combination of an expander and compressor which are directly coupled or interconnected through gears or the like (hereinafter referred to as an expander compressor) in order to efficiently generate a cold (refrigeration) in an air separator. A representative one of techniques of this type is disclosed in Japanese Patent Laid-Open No. 23771/1985. This technique first raises the temperature of gaseous air or nitrogen supplied from a lower column of a double rectifying column by making this gaseous stream pass through a reheat-cycle passage of a main heat exchanger and through a circulating heat exchanger installed separately from the main heat exchanger, pressurizes the heated gas with a compressor, makes it pass through the circulation heat exchanger so as to cool it, and thereafter introduces it into an expansion turbine, thereby providing a cold necessary for the air separator. This technique enables the cold necessary for the system to be generated with reduced gas flow rate through the expansion turbine. It is thereby possible to reduce the unit power consumption of the product gas.
In the arrangement of this technique, however, the gaseous air or nitrogen extracted from the lower column of the double rectifying column is directly supplied to the circulating heat exchanger, and a part of the same is supplied to the circulating heat exchanger through the reheat-cycle passage of the main heat exchanger for the purpose of temperature restoration, thus necessitating the circulating heat exchanger, so that the arrangement become complicated and the cost of installations is increased. Since the gas which is supplied to the compressor of the expander compressor flows through complicated passages of the circulating heat exchanger, the pressure loss and other energy losses caused therebetween are so large that the system cannot work effectively as desired. In addition, the temperature of the gaseous air or nitrogen extracted from the lower rectifying column is very low (about -170.degree. C.), so that the difference between the temperature of this gas and that of the return gas in the circulating heat exchanger is large and, hence, the cold loss at the warm end of the circulating heat exchanger is large, even when the gas is warmed by mixing with its separated part whose temperature is raised through the reheat-cycle passage of the main heat exchanger.